The present invention relates to a semiconductor integrated circuit containing an automatic equalizer or an adaptive digital filter and the circuits associated therewith and, more particularly, to a semiconductor integrated circuit applied to an echo canceler type transmitting device for data transmission over telephone lines.
Where telephone lines or similar circuits are used for voice and data transmission, the transmission line characteristic is known to fluctuate. Thus an automatic equalizer is incorporated in the receiver or in the transmitter, the coefficient of the equalizer being varied in accordance with the transmission line characteristic as it changes.
The prior art automatic equalizer is generally constituted by a transversal filter. The operations of the filter fall into two categories: those for generating a filter output value based on input data and tap coefficients (called output operations), and those for modifying tap coefficients by use of the filter output value, the difference from an ideal filter output value, and the input data (called update operations).
Meanwhile, improvements in the digital signal processor (DSP) technology of late years have made it possible to implement an automatic equalizer using a DSP. Although the typical DSP seeks to improve the efficiency of multiply and add operations, there are limits to the amount of operations that may be performed within a given sampling time. This disadvantage appears to have been minimized by the automatic equalizer proposed in Japanese Patent Laid-Open No. 59-139717. The proposed equalizer is designed to lower the amount of operations per sampling time.
However, the abovementioned prior art automatic equalizer has two disadvantages. One disadvantage is that in its training period, the proposed automatic equalizer provides a slower convergence of tap coefficients than other equalizers which update the coefficients in each sampling period. The other disadvantage is that in a system where the characteristic of the line for transmitting sampled data continuously changes, the follow-up characteristic of the automatic equalizer tends to deteriorate.
Japanese Patent Laid-Open No. 60-206232 discloses a method for updating part of the tap coefficients involved in the initial stage of training and for updating all tap coefficients from the middle stage of training onward. One disadvantage of this method is that because all tap coefficients need to be updated in each sampling period from the middle stage of training onward, the number of the taps with the automatic equalizer must be limited under constraints of the throughput of the digital signal processor.